This invention is concerned the manufacture of gaskets which comprise a steel sheet. The invention is applicable to head gaskets for internal combustion engines and to other types of gasket.
A gasket is intended to seal around a passage which passes between two members, the gasket being inserted between the two members. Thus, in the case of a head gasket, the gasket seals around cylinder bores, coolant passages, and oil passages which pass between the cylinder block of the engine and the head of the engine.
One type of known head gasket comprises a steel sheet, eg of stainless steel. The sheet has various openings which correspond to the cylinder bores, coolant passages etc. The gasket also comprises a resilient bead on each side of the sheet. The resilient beads are formed of elastomeric material and extend around a closed path which surrounds those of the openings which require a seal. Each bead projects beyond the surface of the steel sheet so that it can provide a seal around the openings when the bead is compressed by contact with the head or the block of the engine. In some cases, such beads are secured to the surfaces of the steel sheet, eg by being moulded in situ. In other cases, the beads are received in grooves in the surfaces of the steel sheet. In the case with which the invention is concerned, however, the gasket also comprises an opening through the steel sheet, the opening extending around the closed path in the sheet, and the elastomeric material fills the opening and projects as beads on both sides of the sheet. However, in order to maintain the integrity of the steel sheet, the gasket has connecting webs extending across the opening containing the elastomeric material. The webs serve to connect the portion of the sheet which is enclosed by said path with the remainder of the sheet. In order to prevent said webs from disrupting the performance of the beads, the webs have slots formed in the top and bottom thereof, the slots connecting with the opening on both sides of the web and the resilient material also fills said slots. A gasket of this type is described in U.S. Pat. No. 5,267,740, wherein an elastomeric bead in an opening through a steel sheet is described, there being webs (referred to as xe2x80x9cconnecting strutsxe2x80x9d) extending across the opening.
In a conventional method of manufacturing a gasket of the type with which the invention is concerned, a piece of sheet steel is blanked to the outline shape of the gasket and the various holes for cylinder bores etc are blanked out. Also in such a method, an opening is formed through said piece of sheet steel, the opening extending around the closed path in the sheet which is to be taken by the beads. This opening has connecting webs extending across it at selected points, the webs being formed by interruptions in the opening, ie the steel is not disturbed at the webs. The method also comprises pressing said webs so that said slots are formed in the top and bottom of each web, the pressing operation causing the webs to expand along the opening. The method also comprises filling said opening and slots with resilient material, usually by moulding in situ. The resilient material also forms said sealing beads.
In the above-mentioned method of manufacturing a gasket of the type with which the invention is concerned, the pressing operation on the webs is found to be difficult and to cause excessive wear on expensive tooling.
It is an object of the present invention to provide a method of manufacturing a gasket in which said pressing operation can be carried out more easily and without excessive wear on the tooling.
The invention provides a method of manufacturing a gasket, characterised in that the method comprises applying heat to selected localised areas of a piece of sheet steel so that the structure of the steel is softened only in those areas, and forming an opening through said piece of sheet steel, the opening extending around a closed path in the sheet which passes through said softened areas, the opening having connecting webs extending across it at the softened areas, the webs being formed by interruptions in the opening, the webs serving to connect the portion of the sheet which is enclosed by said path with the remainder of the sheet, the method also comprising pressing said webs so that slots are formed in the top and bottom of each web, the slots connecting with the opening on both sides of the web, the method also comprising filling said opening and slots with resilient material which also forms sealing beads extending along said path and projecting from said opening and slots on both sides of the sheet.
In a method according to the invention, the pressing operation takes place on areas which have been softened by heating (these could be termed xe2x80x9cannealed areasxe2x80x9d) so that it is easier to displace the steel and less wear on the tools occurs. A hardness reduction of up to 50% can readily be achieved.
Preferably, said selected localised areas are heated to at least 700xc2x0 C., eg about 900xc2x0 C., to soften the structure of the steel (or, in other words, to anneal the structure of the steel). It is, of course, necessary to avoid melting the steel or deforming the sheet.
Said heat may be applied by inducing an electrical current in said selected areas. Thus, electromagnets may be arranged in pairs above and below the steel sheet at the selected localised areas and operated to induce an electrical current on the steel sheet.
Alternatively, said heat may be applied by passing an electrical current through said selected areas. Thus, electrodes may be arranged in pairs above and below the steel sheet at the selected localised areas and operated to pass an electrical current through said sheet.
The invention also provides a gasket comprising a steel sheet, the sheet having an opening therethrough, the opening extending around a closed path in the sheet, the opening having connecting webs extending across it which serve to connect the portion of the sheet which is enclosed by said path with the remainder of the sheet, said webs having slots formed in the top and bottom thereof, the slots connecting with the opening on both sides of the web, the gasket also comprising resilient material which fills said opening and slots and forms sealing beads extending along said path and projecting from said opening and slots on both sides of the sheet, characterised in that said webs are formed from steel which has a softer structure than the remainder of the sheet.
There now follows a detailed description, to be read with reference to the accompanying drawings, of a method of manufacturing a gasket which is illustrative of the invention.